1. Field of the Invention
The present invention relates to the field of semiconductor device fabrication. Specifically, the present invention provides a residue-free overlay target which enables precise alignment of lithographic masks or reticles while substantially preventing residue build up at the surface of the semiconductor substrate and eliminating process steps associated with known overlay targets.
2. State of the Art
As is well known, state of the art semiconductor memory and processing devices include multiple layers of electronic features which must be fabricated using multiple process steps. Individual features of state of the art semiconductor devices are generally defined by photolithographic processes wherein a resist is applied over the surface of a semiconductor substrate, or a material layer overlying a semiconductor substrate, and individual device features are patterned into the resist using a mask or reticle. After patterning the resist, the device features are permanently defined in the material layer being processed or the underlying semiconductor substrate by wet or dry etch steps. Advanced processes used to fabricate state of the art semiconductor devices may require as many as fifteen or more separate patterning and etching steps at varying layers during the fabrication process. However, in order to consistently fabricate functional and reliable semiconductor devices, it is necessary that each pattern be accurately aligned over the semiconductor substrate or material layer being processed, and the feature dimensions must be precisely defined at each patterning and etch step. This is particularly true for state of the art semiconductor devices, which require tolerances in the tens of nanometers range.
To help ensure that the device patterns arc accurately positioned, a mask or reticle may be aligned using overlay targets located outside the chip pattern defined on the water being processed. Overlay targets are generally etched into the semiconductor substrate or into an overlying material layer and, therefore, become a permanent part of the water being processed. As new layers are deposited, patterned, and etched, the new masks or reticles used to process the new layers are often aligned by referencing back to the permanent overlay targets previously defined in an underlying material layer. used. In state of the art fabrication facilities, however, automated registration tools are preferred because of their accuracy and high throughput capabilities. To accomplish their task, such automated registration tools must be able to readily detect the edges of the pattern formed by the overlay targets. Yet, as will be described hereinafter, known overlay targets enable intermittent accumulation of process residue which obscures the edges of the overlay target patterns, thereby substantially interfering with the proper function of registration tools.
Known overlay targets generally include a pattern formed by one or more etched trenches or pad areas. Illustrated in drawing FIG. 1 is a portion of an intermediate wafer structure 5 including a simple trench-type overlay target 10. The overlay target 10 includes a continuous rectangular trench 11 etched into the semiconductor substrate 14 outside the chip pattern 15. Of course, the overlay target depicted in drawing FIG. 1 is provided for illustrative purposes only. It is understood that overlay targets can be created using a variety of patterns formed from continuous trenches, discontinuous trenches, or pad areas.
Depicted in drawing FIG. 2 is a cross-section taken at line A-A of drawing FIG. 1, illustrating an overlying material layer 16 deposited over the surface 18 of the semiconductor substrate 14 after formation of the trench 11 defining the overlay target. As can be seen in drawing FIG. 2, the overlying material layer 16 tends to conform to the topography created by the trench 11. Such conformation results in the formation of depressions 20 at the upper surface 22 of the overlying material layer 16. Even after a polishing step, portions 24 (shown in drawing FIG. 3) of the depressions 20 may still remain and serve as collection points for process residue 26 (also shown in FIG. 3), such as hemispherical grain (“HSG”) Poly. As is shown in drawing FIG. 4, because the residue 26 overlies the trench 11 defining overlay target 10, the residue 26 works to obscure the outlines (depicted by dashed lines 28a and 28b) of the pattern formed by the trench 11, making the outlines 28a and 28b of the overlay target 10 to appear ragged or inconsistent. Though drawings FIG. 2 through FIG. 4 depict features associated with a trench-type overlay target, intermittent collection of obscuring residue is also an issue of pad-type overlay targets and overlay targets including one or more trenches or pads
Though the build up of process residue over an overlay target may occur only wafer must be periodically cleaned, such as, for example, by patterning and etching steps, in order to ensure each of the overlay targets formed in the wafer are clean and easily registered by a registration tool. As is easily appreciated, such cleaning steps add time and cost to the fabrication process. Therefore, a overlay target which does not lead to the collection of obscuring process residue would be an improvement in the art, obviating the cost and delay associated with the cleaning steps currently undertaken to ensure the proper registration of overlay targets.